1. Field of the Invention
This invention relates to improvements in a throttle control system for an internal combustion engine, and more particularly such a system for controlling a throttle valve to accomplish a traction control upon learning the fully closed position of the throttle valve.
2. Description of the Prior Art
Hitherto a traction control system for an automotive vehicle has been proposed and put into practical use to improve starting ability and acceleration of the vehicle on a slippery road surface by preventing wheel spin of driving road wheels of the vehicle under controlling engine torque and brake force when the driving road wheels are under a condition to tend to occur the wheel spin. Such a traction control system is disclosed, for example, in Japanese Patent Provisional Publication No. 4-58049.
In such a traction control system, in addition to a first throttle valve opened or closed depending upon an acceleration operation, a second throttle valve is provided and disposed in series with the first throttle valve to be opened or closed by an actuator. The second throttle valve is usually left opened, and compulsorily closed to a target opening degree under a predetermined operating condition for requiring a traction control. Additionally, the traction control system includes a throttle sensor for detecting the opening degree of the second throttle valve. The throttle control system is arranged to feedback-control the second throttle valve to obtain a target opening degree in a high precision, in accordance with a detection result of the throttle sensor. In this traction control system, for the purpose of avoiding lowering in precision of throttle valve opening degree due to non-uniform quality and/or deterioration of the throttle sensor, the second throttle valve is controlled to be compulsorily fully closed immediately after turning ON of a key switch, upon which an output of the throttle sensor at this time is learned as a fully closed position corresponding value, thereby maintaining the precision of a throttle opening degree control using the throttle sensor.
Furthermore, another throttle control system has been proposed as disclosed, for example, in Japanese Patent Provisional Publication No. 3-61654, in which a single throttle valve is used. In this throttle control system, the opening degree of the throttle valve is usually decided in accordance with the operation of an acceleration pedal, and is automatically controlled by an actuator regardless of the operation of the accelerator pedal under a predetermined operating condition requiring a traction control. Also in this throttle control system, a sensor is provided to detect the control amount or the like of the actuator for automatically controlling the opening degree of the throttle valve, upon which the control amount of the actuator is feedback-controlled to a target level in accordance with a detection result of the sensor. For the purpose of avoiding the lowering in precision of the control due to non-uniform quality and/or deterioration of the sensor, the actuator is compulsorily operated to fully close the throttle valve immediately after the turning ON of the key switch, upon which the output of the sensor at this time is learned as a fully closed position corresponding value thereby to maintain the precision of the throttle valve opening degree control by using the sensor.
However, drawbacks have been encountered in the above conventional throttle control systems as discussed below. With the former throttle control system accomplishing the traction control by using the first and second throttle valves, the fully closed position corresponding value (learning value) obtained immediately after the turning ON of the key switch cannot conform to an actual state of a vehicle operating condition in case that each of the first and second throttle valves are provided with a fast idle cam which is adapted to change the fully closed position of the throttle valve in accordance with the engine temperature. This results in a fear of an intended traction control being not able to be made in a high precision.
Also with the latter throttle control system for accomplishing the traction control by using the single throttle valve, the fully closed position corresponding value (learning value) obtained immediately after the turning ON of the key switch cannot conform to an actual state of a vehicle operating condition in case that the throttle valve is provided with a fast idle cam which is adapted to change the fully closed position of the throttle valve in accordance with the engine temperature. This results in a fear of an intended traction control being not able to be made at a high precision.
The above fast idle cam is arranged to compulsorily change the fully closed position of the throttle valve under the action of a wax pellet which is deformable in response to a temperature change in engine coolant of an automotive internal combustion engine.
Now, the above-mentioned learning of the fully closed position of the throttle valve as the fully closed position corresponding value is carried out once at a timing immediately after the turning ON of the key switch (before an engine starting) in order to be made securely and earlier. It will be understood that such a control for compulsorily fully closing the throttle valve provides no influence to engine operation. In case that the fully closed position of the throttle valve is offset to an open side (at which the throttle valve opens wider) in a cold engine condition, the fully closed position shifted under the action of the fast idle cam is unavoidably learned as the fully closed position corresponding value (learning value). Thereafter, even if the control of fully closing the throttle valve in response to the requirement of the traction control is to be made under a state in which the throttle valve can close to its inherent fully closed position upon completion of engine warming-up, the throttle valve can close only to its slightly opened state obtained under the action of the fast idle cam since the fully closing control of the throttle valve is made on an index which is the fully closed position learned at the timing of turning ON of the key switch. This makes impossible to achieve an intended output torque control while applying a large burden to a brake control.